The Ligand Substitution Reactions of Hydrophobic Vitamin _B12 Derivatives. Reaction of Cobyric Acid Heptapropyl Ester with Heterocyclic <it>N</it>-donor Ligands

Abstract

The hydrophobic cobyrinic acid heptapropyl ester corrinoids XCbs-Pr (axial ligand X=CN^–, SO₃^2–, CH₃^– and CH₃CH₂^–) have been prepared from vitamin B₁₂ by hydrolysis of the amide side chains and their conversion to propyl esters. Both the position of the γ-band and the general shape of the UV-visible spectra of these complexes show significant solvent dependence as the polarity of the solvent is varied. The equilibrium constants, K, for the reaction of five-membered heterocyclic nitrogenous bases (the azoles imidazole, pyrazole and 1,2,4-triazole) with displacement of coordinated H₂O in aquacyanocobyrinic acid heptapropyl ester, and coordination by the predominantly five-coordinate complexes sulphitocobyrinic acid heptapropyl ester, ethylcobyrinic acid heptapropyl ester and methylcobyrinic acid heptapropyl ester, have been determined spectrophotometrically at 25 °C in water, methanol, acetonitrile, ethyl acetate and toluene. Values of K are dependent on the identity of the trans ligand (X=CN^–>SO₃ ^2–> CH₃ ^– > CH₃CH₂ ^–); they increase with the basicity of the azole (pyrazole < 1,2,4-triazole < imidazole); and they increase as the solvent polarity increases (toluene<ethyl acetate< acetonitrile< methanol< H₂O). Molecular mechanics calculations suggest that these effects are largely electronic in origin.

Keywords: Hydrophobic vitamin B₁₂, cobalt corrinoids, equilibrium constants, solvent polarity, trans influence

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